Air pollution concerns worldwide have led to stricter emissions standards requiring significant reductions in gas turbine pollutant emissions for both industrial and power generation applications and propulsion applications. Nitrous Oxide (NOx), which is a precursor to atmospheric pollution, is generally formed in the high temperature regions of the gas turbine combustor by direct oxidation of atmospheric nitrogen with oxygen. Reductions in gas turbine emissions of NOx have been obtained by the reduction of flame temperatures in the combustor by a variety of techniques including "rich burn, quick quench, lean burn" and "lean premix" combustion.
Many propulsion engines employ annular combustors to burn fuel. The fuel is metered and sprayed into the combustor through a plurality of fuel nozzles along with combustion air having a designated amount of swirl. The prior art, however, does not disclose a nozzle or the dome end of the combustor configured to mix the fuel and air uniformly to reduce the flame temperatures. Accordingly, non-uniformity of the air/fuel mixture causes the flame to be locally hotter; leading to significantly enhanced production of NOx.
In the typical aircraft gas turbine engine, flame stability and variable cycle operation of the engine dominate combustor design requirements. This has in general resulted in combustor designs with the combustion at the dome end of the combustor proceeding at the highest possible temperatures at stoichiometeric conditions, which in turn, leads to large quantities of NOx.
While premixing ducts in the prior art have been utilized in lean burning designs, they have been found to be unsatisfactory due to flashback and auto-ignition considerations for modern propulsion applications. Flashback involves the flame of the combustor being drawn back into the mixing section, which is most often caused by a backflow from the combustor due to compressor instability and transient flows. Auto-ignition of the fuel/air mixture can occur within the premixing duct if the velocity of the airflow is not fast enough, i.e., where there is a local region of high residence time. Flashback and auto-ignition have become serious considerations in the design of mixers for propulsion engines due to increased pressure ratios and operating temperatures.
Accordingly, there is a need for a fuel preparation chamber that provides a more uniform mixture of fuel and air in the combustor.